1. Field of the Invention
The present invention relates in general to speed change mechanisms of an automatic transmission for a wheeled motor vehicle, and more particularly to the speed change mechanisms of a type used for a power transmission arrangement in which an engine, an oil pump driven by the engine, a speed reduction planetary gear unit for reducing the speed of rotation from the engine and a speed change unit inputting thereinto the speed reduced rotation are coaxially arranged and a direct clutch for directly transmitting the rotation from the engine to the speed change unit is disposed around the speed reduction planetary gear unit.
2. Description of the Related Art
Hitherto, for the purpose of improving fuel consumption and drivability of wheeled motor vehicles, various multi-speed type automatic transmissions have been proposed and put into practical use. One of the speed change mechanisms of such type is shown in Japanese Laid-open Patent Application (Tokkai) 2000-55152.
The speed change mechanism of this published application comprises generally a speed reduction planetary gear unit that reduces the speed of rotation from an engine, a speed change unit that is arranged behind the planetary gear unit and a direct clutch that directly transmits rotation from the engine to the speed change unit. The speed change unit includes second and third planetary gear units, clutches and brakes. By selectively engaging and disengaging the direct clutch, the clutches and brakes, multi-forward speeds and a reverse position are achieved by the speed change mechanism.
For fulfilling the need of reducing the input rotation from an engine, the speed reduction planetary gear unit has a sun gear fixed to a fixed member and makes one of a ring gear and a pinion carrier serve as a rotation input member and the other of them serve as a rotation output member.
The rotation input member is connected to an input shaft of the transmission, and the rotation output member is connected to a selected one of rotation members of the speed change unit through a clutch.
In the above-mentioned speed change mechanism, an oil pump driven by the engine is arranged between the engine and the speed reduction planetary gear unit, the sun gear of the speed reduction planetary gear unit is tightly disposed on a cylindrical boss portion formed on a pump case, and the direct clutch is arranged to surround the speed reduction planetary gear unit. For controlling the clutches and brakes, a hydraulic controller is provided which comprises a control valve body fixed to a transmission case. Within the control valve body, a hydraulic pressure is generated based on a base pressure produced by the oil pump, and the hydraulic pressure is led, through oil passages formed in the pump case, to work chambers of the clutches and brakes to selectively engage and disengage the same.
In the above-mentioned speed change mechanism, an annular piston of the direct clutch is placed near the oil pump and arranged so that an inner cylindrical surface of the annular piston is axially slidably received on the cylindrical boss portion of the pump case having a seal ring interposed therebetween.